New expensive power cord for amp and no change in sound?

Sell it and move on .

Your amp is not mid fi, it is high end, and I think if the amp is technically well constructed and you use a well constructed cable, could be $50-$100 in price depending on enclosure, your amp should sound at its 100%!
If the amp is changing sound signature because of a cable, be it $100 or $5000 then something is wrong with that amp in my opinion.

And I don’t think anyone can hear a difference in a well constructed cable, be it $100 or $5000 in a double blind test.

And saying that one’s ears or system is not good enough (resolving) is just stupid.

I have listened to very resolving systems and these systems mostly can only play the so-called AudioPhile CD’s and one can only listen for a short while before one has to leave the room feeling tired of the sound.

I am not saying that cables are not important, but mostly they just have to be the right length and well made technically and to suit your system, no need to go crazy money vise.

Sometimes I see systems where the cabling cost more than the system! Which just doesn’t make sense, but it’s a free world, so each to his own 👍🏻

I just upgraded all the cables in my system and a new power conditioner.  Spent about $20K.  I would describe the sonic benefit as marginal.  Instead of buying expensive cables I would save my money and buy better speakers.  That's where major improvements can be had.

It might have the same ’sound’ as the fuse, here’s slo blo 3 amp, for a 30 wpc tube amp.

https://www.google.com/shopping/product/7245096972818027189?q=slo+blo+fuse+3.2+amp+120+volt&prds=epd:10311478264179553760,eto:10311478264179553760_0,pid:15108113217225807992&sa=X&ved=0ahUKEwiyipKjl_6FAxXqElkFHWhHAMEQ9pwGCAs

sometimes there are internal fuses and/or relays, rated for ..... not a lot

I've heard power cord shootouts on amplifiers, and differences are, to me anyway, quite subtle and it is hard to really compare cords because of the time it takes to make the switch.  With interconnects and speaker wire, the comparison is MUCH easier to make--you can put one interconnect or cable on one channel and the other interconnect or cable on the other channel and then swing a balance control to easily make a comparison.  I was surprised at how easy it is to hear a difference that way.  

A local dealer who makes custom systems is currently experimenting with full range drivers in Karlson cabinets.  This is at an early stage of development, but, he has been trying different speaker cables.  While I knew from experience that Audio Note silver and copper speaker cables sound quite different, it was still shocking to hear how much different they sound when doing the left-right instantaneous comparison.  Equally surprising was how different listeners reacted to the difference.  Some liked the silver cables more (I personally appreciated the clarity and more explosive dynamics of the silver cable) and others, like me, preferred the copper cables (richer and better tonal balance for this particular speaker; the more attenuated dynamics and liveliness was not a big deficit to me because the speaker is so dynamic).  In my own system, I use silver speaker cables, so if anything, I would normally be biased toward silver; but not in this case.